It has long been known to mount an antenna on top of a vehicle for communicating with a satellite. It is usually required that such antennas be steered in order to track satellite movement, for instance, to acquire new satellites as they come into view of the antenna and to compensate for motion of the vehicle carrying the antenna. To do these tasks antennas are steered either electronically or mechanically. Due to the high complexity and cost, however, fully electronically scanning an antenna is usually not the first (best) choice. More typically, a combination of mechanically and electronically scanning, or particularly fully mechanically scanning an antenna is the preferred choice.
Sometimes an antenna system contains both receiving and transmitting sections in which the relevant polarizations and/or frequencies are different. It may also be desirable that an antenna system have the capability of handling two different types of electromagnetic waves, e.g., circular polarization from TV satellites and linear polarization for internet connection with the same or another satellite.
For a single antenna to serve multiple of these purposes then requires complex feed networks and/or wideband radiating elements. The wideband radiating elements often do not have their best performance in the desired transmitting or receiving frequencies, particularly when used as a reflector antenna feed or elements of an array. For widely separated receiving and transmitting frequencies, e.g., 12 GHz and 14 GHz, relatively poor performance is then quite likely. Furthermore, placing one antenna on top of the other or placing two, or more, antennas side by side also increases the profile or antenna extent in the azimuth dramatically.
A single antenna using an interleaved array configuration of two different arrays may avoid the overall size issue, but increases complexity and manufacturing cost, as it becomes more than simply two antenna arrays. This also reduces the total efficiency by resorting to compact transmission lines which are relatively lossy. Some examples of such antenna designs are found in U.S. Pat. No. 6,028,562 by Guler et al. and its modified version in U.S. Pat. No. 6,127,985 by Guler.
U.S. Pat. No. 6,839,039 by Tanaka et al. discloses an antenna system using two distributed and generally planar arrays, one for receiving and the other for transmitting. Although this approach addresses some issues, it increases the complexity for the generally planar arrays by distributing them as single elements, or as a linear array of elements placed in parallel over a surface and distanced apart to prevent the antenna rows from overshadowing each other. Coherently combining the signals received from the receiver array partitions, and distributing the transmitting power between the relevant partitions over the desired frequency, therefore requires complex feed systems, particularly when there is a need for elevation scanning. On the other hand, the smaller the parts that the antenna is broken down into, e.g., single linear arrays, the larger the overall extension of the total antenna in the azimuth. This is mainly because the wider elevation beamwidth of the smaller parts then requires more distance between the adjacent rows of the elements or linear arrays to reduce their mutual coupling, which is especially an issue when a transmitting row is adjacent to a transmitting row.
U.S. Pat. No. 5,929,819 by Grinberg and Int. App. No. WO 01/11718 by Geller, WO 2004/075339 by Mansour et al., and WO 02/097919 by Collins are additional examples of this approach of resorting to distributed planar arrays. These references teach similar techniques and have similar problems to those discussed, and they do not cover the case of using two different antennas for transmitting and receiving.
In summary, the trend in this art has been to break down single antennas into smaller parts. Breaking down an antenna into similar but smaller antennas is normally done to increase the total gain for a given size constraint, if very low elevation angles are not to be covered. Simulations have indicated that for a given maximum height and diameter (in azimuth extent), the maximum total gain that can be obtained will be through breaking down the antenna into two parts.
Comparing some examples, however, it can be seen that the added gain is often neither considerable nor worth the added complexity and cost. Two cases can be compared, both using a minimum elevation angle of 20 degrees and operation in the Ku-band. For the first case a maximum antenna height of 6 inches and a diameter of 38 inches provides about 1.8 dB in extra gain, as compared to the single antenna with the maximum size to be placed in the same space. For the second case, a maximum antenna height of 5 inches and a diameter of 24 inches provides about 0.7 dB in extra gain. But the calculated benefits here are in an unrealistic ideal situation, i.e., 100% efficiency of the distribution network. Even using a highly difficult-to-obtain 80% efficiency reduces the total gain by about 1 dB, and when scanning in elevation is required over a wide bandwidth the maximum obtainable efficiency is even less, rendering very marginal benefits, if any. This approach of breaking down antennas causes difficult and expensive extra problems, such as a requirement to use a coherent distribution network, and inherently tends to add design and performance compromises.
Accordingly what is needed is a single overall antenna system that is suitable to concurrently handle either or both of two distinct frequency ranges and signal polarizations. This antenna system should preferably be compact and have a low profile, to facilitate mounting with low susceptibility to external forces like wind, rain, hail, etc., and particularly for exterior mounting on vehicles. This antenna system should also be suitable for rotation about at least one axis, driven by an essentially conventional mechanism, to mechanically scan the antenna system.